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The layered oxysulfides Sr2MO2Cu2S2 (M = Mn, Co, Ni) consist of alternating perovskite-type Sr2MO2 layers and copper sulfide layers. We studied the electrochemical insertion of Li into these three samples. By this we were able to study the influence of the nature of the transition metal on the Li insertion process which appears to be at least partially reversible. While the Mn compound clearly shows a Cu-Li exchange reaction, the electrochemical process for the two other compounds is more complex. The lithiated materials were studied by 7Li MAS NMR.
Titanate layered oxide intercalated with hydrated Eu3+ was synthesized by the electrostatic self-assembly deposition (ESD) method. The emission intensity of Eu3+ decreased rapidly with time during irradiation by UV light having energy higher than the band gap energy of the host Ti1.81O4 (TiO) layer. This is presumably due to the decrease in energy transfer from the host TiO layer to Eu3+ as a result of the change in the hydration state of water molecules surrounding Eu3+, which is caused by the hole produced in the TiO valence band. When irradiation was discontinued, the emission intensity gradually recovered. The recovery time increased when the water in the interlayer is removed by heat treatment. This indicates that the state of interlayer water changes during irradiation and returns to its initial state after discontinuation of irradiation. The excitation spectra changed drastically at any given wavelength upon irradiation with UV light. A comparison of the excitation spectra before and after irradiation reveals that only the excitation peak at around the irradiation wavelength decreased upon irradiation, as in the case of spectral hole burning. The hydration state of water molecules surrounding Eu3+ presumably changes depending on the irradiation wavelength.
Various Lamp phosphors, including [Ca10(PO4)6(Cl,F):Sb:Mn], (Y,Eu)2O3 (YOE), BaMgAl10O17:Eu (BAM), and (La,Ce)PO4:Ce:Tb (LAP), with or without flux, have been synthesized by a microwave processing technique in a multimode microwave furnace operating at 2.45 GHz. The microwave-synthesized phosphors were comprehensively characterized for particle size, specific surface area, brightness, and luminescence. Although most properties of the microwave-synthesized phosphors were comparable to that of the conventional products, the kinetics of the phosphor synthesis was substantially enhanced in the microwave processing. As a result, the soaking time at the final temperature was reduced by up to 90% compared to a conventional process. In addition, the required synthesis temperature was also lowered by 100-200°C in microwave process, compared to the conventional process for these lamp phosphors. Certain improved property was also observed in some microwave synthesized samples. The mechanism and advantages of microwave process for the lamp phosphor synthesis through solid-state reaction are addressed.
Epitaxial thin films of Mn3O4 and ZnMn2O4 have been grown hydrothermally on (100) and (111) MgAl2O4 substrates. Film growth was characterized as a function of pH, concentration, and time and thin film X-ray diffraction revealed that the resulting films are an epitaxial continuation of the underlying spinel lattice. Reduction of these films to MnO occurred topotactically and in the case of ZnMn2O4, resulted in mesopores aligned along the <100> directions. As the films maintain an epitaxial relationship with the substrate, the mesopores are aligned macroscopically within a single crystal lattice.
Solid state chemists have long been interested in templated growth of materials using many approaches. The resulting materials have been useful in areas as diverse as photonics and catalysis. Microstructured optical fibers (MOFs) form a new class of nanotemplates that can have sub 20 nm pores that are meters to kilometers long. We have developed a high-pressure microfluidic chemical process that allows for conformal deposition of materials within MOFs to form the most extreme aspect ratio semiconductor nanowires known. The wires can be spatially organized with respect to each other at dimensions down to the nanoscale because the MOF templates can be designed with almost any desired periodic or aperiodic pattern. Many if not most of the chemistries used for conventional chemical vapor deposition (CVD) can be adapted for this process. The resulting materials should enable a large range of scientific and technological applications.
A better understanding of magnetic interactions, charge distribution, and redox properties of vanadium oxide nanotubes (VONTs) is necessary for accurate structure and mechanism of formation determination. Magnetic properties have been determined for pristine and lithiated VONTs and for the VONTs arranged in nanourchin morphology. Presence of paramagnetic V4+ ions and V4+ ions coupled in magnetic dimers is found, and their amounts are estimated. Both lithiation and change of morphology to nanourchin destroy the spin-gap behavior, which indicate changes in charge distribution. No ferromagnetic response is observed in lithiated VONTs. Magnetic properties of vanadium oxide nanorods with δ-V4O10 structure are also characterized.
The structure and dielectric properties of rare earth niobate compounds within the Ln3NbO7 (Ln = Nd, Gd, Dy, Er, Yb and Y) and Ln2(Ln',Nb)O7 (Ln = Nd, Sm and Ln' = Yb) series are investigated. The crystal structure of the all the studied materials is found to be fluorite-related including webertite-type, pyrochlore, and defect fluorite structures. It is observed that the relative permittivity of the defect fluorite Ln3NbO7 (Ln = Dy, Er, Yb and Y) increases with the increase in temperature and exhibits low dielectric loss up to approximately 350 K. Above 350 K, the dielectric loss increases rapidly with increasing temperature as the onset of electrical conductivity takes place. Of particular interests are Gd3NbO7 and Nd3NbO7, which exhibit a frequency and temperature dependent dielectric relaxation behavior. At 1 MHz Gd3NbO7 reaches its maximum relative permittivity of ∼34 at about 330K, while at the same frequency, the maximum relative permittivity of Nd3NbO7 is attained at about 500 K. By contrast, Nd2(Yb,Nb)O7 and Sm2(Yb,Nb)O7, which crystallize in a pyrochlore-type structure, do not show dielectric relaxation and, comparatively, exhibit a more temperature-stable dielectric permittivity response.
Clathrates of type I to type III are presented from the two points of views. The first one is intriguing electronic states appearing from the variation in phonons. The various phonons ranging from lattice phonons, intra-cluster phonons and atomic phonons are described in connection to the electronic states created via electron-phonon interactions. The other issue is the application to thermoelectric power materials on a basis of the concept of phonon-glass-electron-crystal (PGEC).
Multistep topochemical reactions can be used to construct alkali-metal halide arrays within layered perovskite hosts. Combinations of ion exchange and reductive intercalation (A = Li) or reductive and oxidative intercalation (A = Rb) allow one to prepare the compounds such as (A2Cl)LaNb2O7. These products consist of perovskite blocks separated by double alkali-metal halide layers where the local layer structure is dependent on the size of the alkali cation. Details on the synthesis and structures of these materials are presented, and the general utility of the topochemical strategies used in their preparation is discussed.
We have synthesized a series of nanocrysatlline (5±3 nm in diameter) and submicrom ZnS:Ag,Al phosphors with various dopant compositions via a newly developed emulsion method. The X-ray diffraction (XRD), EDS, SEM, and TEM were utilized in the characterization of phase purity and microstructure of phosphor particles. Photoluminescence (PL) and cathodoluminescence (CL) spectra were also utilized to characterize the optical properties of blue-emitting phosphor. The CL intensity of submicron ZnS:Ag,Al phosphor was found to be weaker than that of corresponding commercial product, which was attributed to the poor crystallinity and small grain size. In this article we described the synthesis of ZnS:Ag,Al nanophosphor with emphasis on the correlation between spectroscopic features and the microstructure.
The growth of new phases out of high temperature hydroxide solutions as a means of discovering new materials is discussed. We have succeeded in solubilizing rare earth cations and platinum group metal cations in molten hydroxides and have grown single crystals with a large number of new compositions and new structure types. The use of sealed silver tubes has enabled us to control the water content and, hence, the acidity of the hydroxide melts, and thereby to grow crystals via slow cooling. The synthetic conditions and structures of several new oxides including Ln1-xNa1+xIrO4 (Ln = Gd-Er, Y; x = 0.04-0.26), Ln3RuO7 (Ln = La, Sm, Eu), LnNaPd6O8 (Ln = Tb-Lu, Y) and La9RbIr4O24 are presented.
A “toolkit” of emulsion templating and directional freezing methods has been developed which allows the preparation of a wide variety of organic, inorganic, and metallic materials in a macroporous or hierarchically porous form.1,18,20 The various processes use water, organic solvents, or aqueous/organic emulsions as the template phase. We have shown that the organic solvent can be replaced by liquid CO2 in both the emulsion templating and directional freezing approaches, thus reducing organic waste and offering advantages in applications such as the preparation of biomaterials.
The Seebeck coefficients of several ruthenates which structures derive from the perovskite, have been measured up to 800K. All the values above 300K are found to be in the range +25μV.K-1 to +35 μV.K-1, with a very weak dependence on both T absolute value and electrical resistivity. This demonstrates that S at high T depends mainly on the spin degeneracy term of the modified Heikes formula. The insensitivity of thermopower to the chemical doping confirms the applicability of the model. The present study also shows the good properties of ruthenates as thermoelectric p-type oxides for high T energy conversion.
We have examined the ability of a carefully chosen perovskite, BaCeO3, to act as a redox host for noble metals, notably Pd, in the hope of producing an “intelligent” catalyst where palladium is absorbed into the host lattice as ions under oxidative conditions and released as elemental Pd under reducing conditions. Pd-substituted perovskites BaCe1-xPdxO3-δ (0 ≤ x ≤ 0.1) were prepared by solid-state reactions in pure oxygen. The crystal structure was refined in the orthorhombic space group Pnma. Palladium was found to be driven in and out of the perovskite lattice upon repeated redox cycles by detailed XRD study. SEM/EDX revealed the formation of perovskite nanowires and nanorods when reducing Pd-substituted BaCeO3 up to 1000°C.
In this report, we present a bio-inspired encapsulation process to create nanocluster-assembled core-shell particles under aqueous, room temperature and non-toxic conditions. The approach to synthesize calcium carbonate core-shell particles is accomplished by employing a Polymer-Induced Liquid-Precursor (PILP) process. We demonstrate the amorphous mineral precursor is coated around a core of hydrogel nanoparticles, and subsequently solidified and crystallized. The synthesized core-shell particles are 300∼500nm diameter and ∼100 nm shell-thickness. We investigate the role of the hydrogel core of the particle using time-resolved XRD, thermal-XRD and thermal analysis. The organic hydrogel appears to influence the transformation of mineral phases, stabilizing the amorphous phase of calcium carbonate.
Nanocrystalline ceria-zirconia samples doped with rare-earth (Gd, Pr, Sm, La) cations were prepared via modified Pechini route. Effect of their real structure and surface composition characterized by a combination of sophisticated physical methods (XRD, TEM +EDX, EXAFS, WAXS, UV-Vis, XPS, SIMS) on the mobility and reactivity of the lattice oxygen estimated by oxygen isotope exchange, H2, CH4 and CO TPR was analyzed. For the reaction of acetone autothermal reforming into syngas, catalytic activity correlates rather well with the oxygen mobility controlled by the type and content of a dopant
The layered niobium oxysulfide was synthesized by the heat treatment of K4Nb6O17 · 3H2O layered oxide in the mixture of H2S/N2 gases. The layered oxysulfide had the large plate-like shape, and the total ratio of compound was estimated KxNbS2-yOy (The number of x was from 0.2 to 0.4, y was from about 0.5 to 1.0). According to the XPS measurement, the oxidation state of Nb was estimated for +2, +4, +5, and oxidation state of S was -2. It was confirmed that the layered oxysulfide has ion exchange, and intercalation capabilities as proton exchange reactions take place in H2SO4 solution and bulky amine molecules can be intercalated in an amine solution.
The perovskites PbFeO2F and 0.5PbFeO2F-0.5PbTiO3 were synthesized at high temperatures (1000°C) and high pressures (4 – 6 GPa). The crystal and magnetic structures were determined using powder neutron diffraction. Quenched PbFeO2F has the cubic perovskite-type, Pm3m, structure in which the Pb ion shifts from ideal A-site along the <110> directions, which is in good accordance with a previous report. The magnetic structure is antiferromagnetic G-type with propagation vector k = (1/2 1/2 1/2) and an Fe3+ ordered moment of 3.83 μB at 283K. The Néel temperature is 655(5) K. Annealed PbFeO2F has a tetragonal perovskite-type structure at room temperature and transforms reversibly from tetragonal to cubic at approximately 470 K. A superlattice with dimensions a × a × 5c is observed both in electron and x-ray diffraction. The solid solution 0.5PbFeO2F-0.5PbTiO3 belongs to the non-centrosymmetric space group P4mm. The magnetic structure is G-type antiferromagnetic and shows a weak ferromagnetic moment at 4 K. Consequently, 0.5PbFeO2F-0.5PbTiO3 is simultaneously ferroelectric and a weak ferromagnet at low temperature. The Néel temperature is 450 K but the temperature dependence of the ordered Fe moment is anomalous.
The synthesis of MS2 (M = Mo, W) onion-like nanoparticles by means of a high temperature MOCVD process starting from W(CO)6 and elemental sulfur is reported. The reaction can also be carried out in two steps where the intermediate amorphous WS2 nanoparticles formed through the high temperature reaction of tungsten and sulfur in the initial phase of the reaction are isolated and converted in a separate annealing step to onion-type WS2 nanoparticles. Based on a study of the temperature dependence of the reaction a set of conditions could be derived where onion-like structures were formed in a one-step reaction. Onion-like structures obtained in the single-step process were filled, whereas the particles obtained by the two-step procedure were systematically hollow. A model could be devised to rationalize the different outcome of the reactions. The MOCVD approach therefore allows a selective synthesis of open and filled fullerene-like chalcogenide nanoparticles. Furthermore, we demonstrate the novel surface functionalization of WS2 nanotubes with polymeric ligands by complexation with a combination of Ni2+ via an scorpionate-type nitrilotriacetic acid (NTA) and immobilization of TiO2 nanoparticles onto the surface of nanotubes. Synthesis of such a functional polymeric ligand was achieved via a reactive polymer precursor route.
By applying directional pressure along the (111) crystal axis of opaline photonic crystals under controlled temperatures, inverse opals with symmetry broken structures are fabricated. This selective deformation results in strongly modified photonic band structures and hence optical properties of the photonic crystals. Experimental data are accompanied by theoretical band structure calculations that confirm the experimental results and are used to predict new structures with optimized band gap properties.